Electrolytic high-speed deposition of aluminum on continuous products

ABSTRACT

The invention is directed to an electrolyte for the electrolytic high-speed deposition of aluminum on continuous products, containing an organometallic aluminum complex of formula (I) 
     
       
         MF.2Al(C 3 H 7 ) 3 .nAlR 3   (I), 
       
     
     wherein 
     M=K, Rb, Cs, 
     R=a C 3  alkyl group or a mixture of a C 3  and a C 1 -C 2  alkyl group, 
     n=from 0.1 to 1, 
     in an aromatic or aliphatic hydrocarbon as solvent.

The invention relates to an electrolyte for the electrolytic high-speeddeposition of aluminum on continuous products, which electrolytecontains an organometallic aluminum complex. The invention is alsodirected to the use of said electrolyte in the production ofcorrosion-resistant and decorative coatings on continuous products in acontinuous process.

By aluminizing base metals, it is possible to make themcorrosion-resistant and provide them with a decorative coating.Optionally, such a coating may also be colored. The aluminum ispredominantly deposited by electroplating from electrolytes enablingsuch an electrodeposition. Amongst the electrolytes are fused-saltelectrolytes as well as electrolytes containing aluminum halides oralkyl aluminum complexes. Electrolyte systems based on alkyl aluminumcomplexes have gained general acceptance in the art. In general, suchalkyl aluminum complexes also contain alkali complex compounds orammonium complex compounds.

Initially, electrolyte solutions containing the NaF.2AlEt₃ complexdissolved in aromatic hydrocarbons such as toluene or xylene have beenused almost exclusively in the electrodeposition of aluminum. However,one drawback of these electrolytes has been their very poor throwingpower which, in particular, has disadvantageous effects when coatingparts of complicated shape as rack products or drum products. With largeparts of complicated shape having angles and corners, the poor throwingpower results in incomplete and non-uniform coating.

In the course of time, therefore, electrolyte systems have been employedcontaining potassium halides instead of sodium halides. Potassiumhalides exhibit superior throwing power and have compositions such asKF.2AlEt₃. Furthermore, the complexes have superior electricalconductivity compared to the corresponding sodium salt complexes.

One major drawback, however, is the poor solubility of these complexesin aromatic hydrocarbons generally used as solvents, so that the common3-4 M toluene solutions of these complexes already undergocrystallization at 60-65° C., posing a serious problem when aluminizingrack products. Further dilution of these solutions results in a massivedecrease in conductivity and current density resistance, rendering thecoating process uneconomic.

The use of potassium fluoride complexes containing triisobutyl aluminumas complex component has neither provided a substantial solution tothese problems. Complexes of the composition KF.2Al(iBu)₃ have asubstantially lower melting point of from 51 to 53° C., which is lowerthan that of the corresponding ethyl or methyl aluminum complexes. Evenat room temperature and a dilution of 3-4 M in toluene, the isobutylcomplexes do not crystallize. One major disadvantage of this compound,however, is to be seen in its poor current density resistance. Even atlow current densities, gray coatings are formed on the objects to becoated, and there is undesirable co-deposition of potassium.

EP-A 0,402,761 and U.S. Pat. No. 4,417,954 describe prior art methodsintended to solve these problems. To this end, the potassium-containingtriethyl aluminum complexes used to date are to be mixed with otheralkyl aluminum complexes. Such mixtures have lower melting pointscompared to pure triethyl aluminum complexes. In addition, they have ahigher solubility in aromatic hydrocarbons. Triisobutyl aluminum andtrimethyl aluminum are exemplified as admixtures. The compositionsobtained in this way are acceptable for rack product aluminizing withrespect to electrical conductivity, solubility and throwing power andare used on an industrial scale today.

Likewise, the EP-A 0,084,816 describes electrolytes for theelectrodeposition of aluminum, wherein mixtures of aluminum alkylcomplexes are used. According to the examples of this document, mixturesof triethyl aluminum and isobutyl aluminum are used, in particular.

However, such electrolytes are disadvantageous as they are not suitablefor the continuous coating of continuous products such as wires, tapes,long-profiles, or pipes. Such a process and a corresponding device forthe electrodeposition of aluminum on continuous products are describedin the German patent application by the present applicant filedsimultaneously with the present application.

The electrolytes for the electrodeposition of aluminum available up tonow have a low current density resistance of only from 0.2 to 2.0 A/dm²at maximum. When exceeding the maximum limiting current density for aspecific composition, the result will be burns, rough coatings andundesirable co-deposition of potassium. In particular, this is the casewhen adding larger amounts of triisobutyl aluminum as is the conceptionin EP-A 0,084,816 or EP-A 0,402,761, for example.

To date, continuous products such as wire are generally coatedcontinuously for corrosion protection by applying a zinc coating,wherein the galvanizing technique is used. However, this is nohigh-quality corrosion protection because the protective coatingundergoes changes even after a short period of time, forming voluminouswhite corrosion products on the surface as a result of oxidation of thecoated zinc layer. For many applications, there is a demand for a higherquality corrosion protection which can be achieved by usingelectrodeposition of aluminum. Such a coating remains substantiallyunchanged and therefore provides a higher quality corrosion protectioncompared to zinc coating used so far. However, the preconditions for aneconomic production are that the electrolytes employed can be operatedat high current density and quantitative yield, have a long servicelife, are cheap in production and easy to maintain.

The previously known electrolytes for the electrodeposition of aluminumare not suitable for use in such a process, as the requirements for anelectrolyte in continuous coating are essentially different from thosein the previously known rack product aluminizing. In the continuouscoating of continuous products such as wires, tapes, long-profiles, orpipes, the parts to be coated are simple in geometry. The electrode gapsare equal in most of the cases, so that the macro throwing power of theelectrolyte plays a minor role. In contrast to rack product aluminizing,the main requirement in using the electrolyte is a deposition rate ashigh as possible, where sufficient purity and a compact structure of thedeposited layer must be achieved so that, in addition, an electrolytehaving a high limiting current density is required.

It was therefore the technical object of the invention to provide anelectrolyte which has the properties required for the electrolytichigh-speed deposition of aluminum on continuous products, particularly ahigh deposition rate, a high limiting current density, permits operationwith quantitative yield, has a long service life, is cheap in productionand easy to maintain.

Said object is achieved by using an electrolyte containing anorganometallic aluminum complex of formula (I)

MF.2Al(C₃H₇)₃.nAlR₃  (I),

wherein

M=K, Rb, Cs,

R=a C₃ alkyl group or a mixture of a C₃ and a C₁-C₂ alkyl group,

n=from 0.1 to 1,

in an aromatic or aliphatic hydrocarbon as solvent.

To date, such an electrolyte compound has not been used in theelectrodeposition of aluminum and, in particular, has not been usable inrack product aluminizing. In principle, tri-n-propyl aluminum ortriisopropyl aluminum may be used as tripropyl aluminum complex.Particularly preferred, however, is the use tri-n-propyl aluminum.

Furthermore, it can be inferred from formula I that the electrolyteaccording to the invention also comprises alkyl aluminum admixtureswhich are possible in addition to the 1:2 complex. Surprisingly, it hasbeen found that this results in higher values for the applicablelimiting current density and in a reduction of the macro throwing powerwhich, however, is of minor importance in the high speed deposition oncontinuous products.

It is preferred that MF in formula I be KF or CsF. In accordance withformula I, a tripropyl aluminum is provided as further component at amolar ratio relative to MF of 2:1. Preferably, tri-n-propyl aluminum isused. Furthermore, the electrolyte includes a non-complexed trialkylaluminum at a MF/AlR₃ molar ratio of from 1:0.1 to 1:1, withtri-n-propyl aluminum being used in this case or mixtures oftri-n-propyl aluminum with triethyl aluminum at a ratio of from 1:10 to10:1. The electrolyte thus composed is preferably dissolved in anaromatic hydrocarbon such as toluene or xylene, where from 1 to 4 molesof solvent per mole MF are preferably used. It is particularly preferredto use toluene or xylene as aromatic hydrocarbons.

Furthermore, suitable inhibitors may be added to achieve a more compactstructure in the deposition at high current densities. To this end,aromatic or aliphatic ethers, especially anisole or methyl tert-butylether are preferably used.

Such an electrolyte is suitable for use in an electrolytic high speeddeposition of aluminum on continuous products such as wire, tapes,long-profiles or pipes, where the aluminum can be deposited at highcurrent densities of more than 2 to 20 A/dm².

The electrolyte solution of the invention is prepared in a conventionalmanner. First, the metal fluoride is added to the solvent mixture ofhydrocarbons and an optional inhibitor. Thereafter, the amount of alkylaluminum compound calculated for complex formation is added slowly insmall portions. The addition is followed by heating, and stirring untilall the components are completely dissolved. The solution is then cooledto room temperature and may be stored for any period of time.

For the first time, the electrolyte solution of the invention permits ahigh speed electrodeposition to be performed at current densities ofmore than 2 A/dm², where high-quality coatings are obtained. It ispossible to operate at high current densities, and the electrolyte canbe used up to quantitative yield. The electrolyte has a long servicelife, is cheap in production and easy to maintain.

The following examples are intended to illustrate the invention in moredetail.

EXAMPLE 1 Preparation of the Electrolyte Solution

In a heatable stirred vessel, an electrolyte having the compositionKF.2Al(C₃H₇)₃.0.3Al(C₃H₇)₃.0.3Al(C₂H₅)₃.3 moles of toluene was preparedunder argon. To this end, the calculated amount of solvent was chargedfirst into the stirred vessel flooded with argon. Then, the potassiumfluoride previously dried at 120° C. was added with vigorous stirring.Subsequently, the calculated amounts of tripropyl aluminum and triethylaluminum were added slowly in small portions, and the solution heated toabout 80° C. Thereafter, the solution was heated until all thecomponents had completely dissolved and then cooled to room temperature.An entirely fluid, clear solution was obtained.

EXAMPLE 2

Two aluminum anodes of 150×40 mm were positioned in a heatablecylindrical glass vessel of about 3 liters capacity equipped with aglass cap. Between the two anodes, a cylindrical copper cathode of 25 mmin diameter and 100 mm in length was fixed in the glass cap through arotatable cathode bushing.

A coating process was carried out in the above-described vessel, usingan electrolyte having the compositionKF.2Al(C₃H₇)₃.0.3Al(C₃H₇)₃.0.3Al(C₂H₅)₃.3 moles of toluene. Followingcleaning of the cathode, a 11-12 μm thick, compact, bright-whitealuminum layer was deposited at a current density of 8 A/dm² D.C. and95° C. within 7 minutes. During this period, the cathode was rotated ata speed of 400 rpm.

EXAMPLE 3

The electrolyte solution from Example 1 was concentrated to 2.5 molestoluene dilution. Subsequently, 0.5 moles of anisole per mole KF wasadded to the electrolyte. Likewise at 8 A/dm² and with polar reversalcurrent, an aluminum layer about 12 μm in thickness was deposited inthis electrolyte. The electrode motion (rotation) was left unchanged tobe 400 rpm. The generated coating was finely crystalline, bright-whiteand semi-glossing.

EXAMPLE 4

In a test cell of about 6 liters capacity equipped with a lock systemand flooded with Argon, a ring of steel wire 3 mm in thickness having adiameter of 100 mm was coated between 2 anode plates of about 150×150mm. The electrolyte was KF.2Al(C₃H₇)₃.0.2Al(C₃H₇)₃.0.6Al(C₂H₅)₃.3.5toluene. Coating was performed at 6 A/dm², 100° C. and with polarreversal current. The electrolyte was intensively stirred by directingan argon stream through the test cell during the coating process. Thegenerated coating was about 12 μm thick, from matte to satin-like,finely crystalline and bright-white. The cathode yield was 99.6%.

What is claimed is:
 1. An electrolyte for the electrolytic high-speeddeposition of aluminum on continuous products, containing anorganometallic aluminum complex of formula (I) MF.2Al(C₃H₇)₃.nAlR₃  (I),wherein M=K, Rb, Cs, R=a C₃ alkyl group or a mixture of a C₃ and a C₁-C₂alkyl group, n=from 0.1 to 1, in an aromatic or aliphatic hydrocarbon assolvent wherein the electrolyte contains from 1 to 4 moles of solventper mole MF.
 2. The electrolyte according to claim 1, characterized inthat an aromatic or aliphatic ether is contained as inhibitor.
 3. Theelectrolyte according to claim 1, characterized in that R is a mixtureof C₃ and C₂ alkyl groups at a ratio from 1:10 to 10:1.
 4. Theelectrolyte according to claim 1, characterized in that anisole iscontained as inhibitor with 0.1-1 times the amount relative to MF fromformula (I).
 5. The electrolyte according to claim 1, characterized inthat an aromatic hydrocarbon is contained as solvent.
 6. The electrolyteaccording to claim 5, characterized in that the solvent comprisestoluene.
 7. Use of the electrolyte according to claim 1, for theelectrolytic high speed deposition of aluminum on continuous products.8. The use of claim 7, characterized in that the continuous products arewire, tape, long-profiles, or pipes.